This invention relates to cellular mobile communication and to a base station and a mobile unit for use in the cellular mobile communication.
In general, a cellular mobile communication network has an overall service area divided into a plurality of cells and is assigned with a plurality of radio channels. Each channel is reused in different cells which are sufficiently remote from one another. Such a reuse of the radio channels is effective in raising an efficiency of use of a frequency band of such radio channels. The mobile communication network is, for example, an automobile communication network or a portable telephone network and comprises a great number of mobile units.
Usually, a base station is placed in each cell. A mobile unit is movable from one to another of the cells and may currently be present in one of the cells in which is placed the base station under consideration which will be called herein a particular cell merely for brevity of the description. To a call originating either in the base station or in the mobile unit, the base station allocates one of the radio channels as a communication channel for use in communication between the base station and the mobile unit. The communication channel is allocated either by a fixed channel allocation scheme or by a dynamic channel allocation scheme.
According to the fixed channel allocation scheme, the radio channels are preliminarily allocated to the cells in consideration of propagation characteristics of radio signals of respective radio channels so as to give rise to little interference to communication channels in respective cells. The fixed channel allocation scheme is used in the automobile communication network and in the portable telephone network.
In accordance with the fixed channel allocation scheme, a reuse distance of each radio channel is determined primarily based on a carrier to interference ratio (C/R) in a cell edge adjacent in each cell to contiguous cells. An improved fixed channel allocation method is described by William C. Y. Lee in "Mobile Cellular Telecommunications Systems" published 1989 by McGraw-Hill Book Company, Subchapter 8.5 Fixed Channel Assignment. This technique is called reuse partitioning.
According to the dynamic channel allocation scheme, a certain one of the radio channels is selected so as to avoid interference caused to the communication channel by other channels which are currently used in communication in nearby cells. Although the manner of selection is somewhat complicated, all radio channels of the mobile communication network can be used unless the interference is objectionable. It is consequently possible to concurrently deal with a great number of communications. The dynamic channel allocation scheme is therefore prosperous for use in the automobile communication network, the portable telephone network, an outdoor cordless telephone network, and a like mobile communication networks.
An example is disclosed by Toshihiko Kanai in Japanese Patent Prepublication (A) No. 351,126 of 1992 and in a paper he contributed to the Conference Record of the Forty-second IEEE Vehicular Technology Society (VTS), pages 782 to 785, under the title of "Autonomous Reuse Partitioning in Cellular Systems". Another example is revealed also by Toshihiko Kanai in Japanese Patent Prepublication (A) No. 110,500 of 1993. In this latter example, the communication channel is switched from one to another of the radio channels when the interference is objectionable.
In accordance with the autonomous reuse partitioning, an identical ordering of channels is used in common to the cells. As the communication channel, one of the radio channels is allocated that has a carrier to interference ratio exceeding first in the identical ordering an allocation reference level capable of allowing the reuse of radio channels in the particular cell. No cooperative control is necessary. That is, the base station transmits its radio signals through such communication channels with no control on a base station power. This undesirably results in frequent call losses and forced interruptions of the communication channels, namely, forced cut offs of messages being transmitted through the communication channels.
Still another example is proposed in a paper contributed by Ken-ichi Ishii, one of the present joint inventors, and another to the Record of the Forty-fourth IEEE VTC (Vehicular Technology Conference), 1994, pages 838 to 842, under the title of "Dynamic Channel Allocation Algorithm with Transmitter Power Control". According to this Ishii et al paper, one of the radio channels is allocated as the communication channel by merely applying to the carrier to interference ratio a first allocation threshold level equal to a sum of a power control range and a second allocation threshold level which allows the reuse of radio channels in the particular cell like the allocation reference level described above. If this one of the radio channels is not available, another of the radio channels is selected that has a carrier to interference ratio greater than the second location threshold level.
In accordance with the Ishii et al paper, no attention is directed to the identical ordering. The power control range is therefore wide. In fact, the Ishii et al paper refers to power control ranges at the base station and the mobile unit.